Recently, a variety of proposals have been made for a technology to improve a resolution, a sampling frequency or the like of an image signal or an audio signal. For example, in the case of up-converting a standard TV signal corresponding to a standard resolution or a low resolution into a so-called HDTV signal with a high resolution or the case of performing sub-sampling interpolation, it is known that better performance-wise results can be obtained by performing conversion processing involving class categorization rather than performing a conventional method by means of linear interpolation (see Japanese Patent Application Publication No. Hei 7-95591 and Japanese Patent Application Publication No. 2000-59740).
This conversion processing involving class categorization relates to a technology such that, for example, when converting a standard TV signal (SD signal) corresponding to a standard resolution or a low resolution into a signal (HD signal) corresponding to a high resolution, a class to which pixel data at a target position in the SD signal belongs is detected and, by using coefficient data that corresponds to this class, pixel data of the HD signal corresponding to the target position in the SD signal is produced from multiple items of pixel data of the SD signal based on an estimate equation. The coefficient data used in this conversion processing involving class categorization is determined by learning such as the least-square method for each of the classes beforehand.
It is to be noted that the coefficient data used in that conversion processing involving class categorization has been based on a result of learning without performing any categorization in accordance with whether a dynamic range is large or small, which dynamic range is a difference between a maximum value and a minimum value of multiple items of pixel data that constitute a class tap.
In this case, in view of a structure of the least-square method, such coefficient data is created as to reduce an error at a portion with a higher frequency, that is, a portion with a smaller dynamic range. Therefore, at a portion with a larger frequency, that is, a portion with a lager dynamic range, an error with respect to a true value of the pixel data of the HD signal calculated by the estimate equation is liable to be smaller.